在 Node.js 中使用 Jest 进行单元测试

Question

I'm very new to unit testing and very excited about it. But I've uncertainty whether I should add defensive programming style or not to my production code. So, please see the following codes.

calclateFinalYearGPA.js (production code)

const _ = require('lodash');
const getGPAPoint = require('./getGPAPoint');

module.exports = function (gradings) {
  let totalPoint = 0;

  gradings.forEach((grading) => {
    const grade = grading.grading.grade.name;
    const point = getGPAPoint(grade);
    totalPoint += point;
  });

  const finalYearGPA = _.round(totalPoint / gradings.length, 1);

  return finalYearGPA;
};

The gradings is an array of objects which is pulled from the db, with the following structure.

"gradings": [
    {
        "grading": {
            "gradeId": 3,
            "grade": {
                "gradeId": 3,
                "name": "A-"
            }
        }
    },
    ...
]

In my calculateFinalYearGPA.js , I loop through that array and extract grading.grade.name which is A- in the above structure, and pass it to getGPAPoint . It will return the decimal number.

So, now in my calculateFinalYearGPA.test.js test, I've add the following code which is test whether it fails when gradings is not an array.

const calculateFinalYearGPA = require('../../../utils/calculateFinalYearGPA');

describe('calculateFinalYearGPA', () => {
  it('should throw an exception if input is not an array', () => {
    const inputs = [1, 'a', null, undefined, {}];
    inputs.forEach((i) => {
      expect(() => {
        calculateFinalYearGPA(i);
      }).toThrow();
    });
  });
});

So, now I need to add some defensive programming code to my production code.

if (gradings.constructor !== Array)
    throw new Error(`Expected to get an array, got ${typeof gradings}.`);

The way I'm doing is correct? Should I test the gradings array has the proper structure as well? If so, I need to add some code that nothing with business logic is done but check the gradings array's structure. My production code will populate with that kind of code. Is it supposed to be?

What is the proper ways to test my calculateFinalYearGPA.js ?

Answer 1

Actually I don't think that correct, I mean to expect it will throw an error , throwing error logically means that something unexpected happened

so the right way to test the structure of the input is within calculateFinalYearGPA.js file, like the following:

module.exports = function (gradings) {

   if(!Array.isArray(gradings)) {
      throw new Error(`Expected to get an array, got ${typeof gradings}.`);
   }
  let totalPoint = 0;
  ...
};

But in case you need to handle the error in jest or you might check it at least, I just recommend you to create a custom Error class, so you can manipulate as you wish, so:

class CustomError extends Error {
  constructor(code, message) {
    const fullMsg = message ? `${code}: ${message}` : code;
    super(fullMsg);
    this.name = code;
    this.code = code;
    this.message = fullMsg;
  }
  
  toString() {
    return this.message;
  }
}

Note: excpect(SOME_THING).toThrow() in jest will catch any error, so you have to be sure which error would fail the test and which not.

so now we have inside calculateFinalYearGPA.js file

module.exports = function (gradings) {

   if(!Array.isArray(gradings)) {
      throw new CustomError(799, `Expected to get an array, got ${typeof gradings}.`);
   }
  let totalPoint = 0;
  ...
};

Finally for testing

const calculateFinalYearGPA = require('../../../utils/calculateFinalYearGPA');

describe('calculateFinalYearGPA', () => {
  it('should throw an exception if input is not an array', () => {
    const inputs = [1, 'a', null, undefined, {}];
    inputs.forEach((i) => {
      try {
          calculateFinalYearGPA(i);
        } catch(e) {
         expect(e.code).toBe(799); // 799 is an optional number which refers the type mis-matched 
       }
    });
  });
});